I want to add largest_sum_contiguous_subarray in CPP

[varunvilva]

This issue is about [largest_sum_contiguous_subarray[CPP]].

• [x] Issue Title is in form largest_sum_contiguous_subarray [CPP].
• [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• [x] This Algo/DS is worth adding to this repository.
• [x] Assign this issue to me (I want to work on this).
  • [x] I ensure that I am not already assigned to 2 or more issues.
• [x] I will send my PR only after I'm assigned to this issue by any maintainer.
• [x] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.